A byproduct of internal combustion engines, including diesel engines, are various emissions such as nitrogen oxide emissions including NO and NO2, sometimes represented as NOx. In response to increased government-mandated regulations over such emissions, manufacturers of internal combustion engines have developed measures to reduce the amount or effect of the nitrogen oxides produced by the internal combustion process. Two such measures are described in U.S. Patent Application Publication No. 2011/0022068 (the '068 publication).
The '068 publication describes selective catalytic reduction (SCR) methods in which the quantity of nitrogen oxides in the exhaust gasses are reduced through chemical reactions that occur in the presence of a catalyst disposed in the exhaust system. In a typical SCR process, a reductant agent is introduced to the exhaust gasses generally upstream of the catalyst whereat the reductant agent and nitrogen oxides react to convert the nitrogen oxides to other chemicals such as nitrogen (N2) and water (H2O).
Hydrocarbons are one common type of reductant agent that may be used to perform selective catalytic reduction. In some embodiments, the hydrocarbons can be conveniently obtained from the same fuel source used by the internal combustion engine, thereby simplifying the SCR system. However, the specific catalytic materials typical used to perform Hydrocarbon Selective Catalytic Reduction (HC-SCR) can be susceptible to deactivation due to sulfur in the exhaust gasses, which may form sulfur oxides on the catalytic material thereby reducing the number of active sites available on the catalyst to initiate conversion of the nitrogen oxides. To remedy sulfur deactivation, the HC-SCR catalyst may be occasionally regenerated by heating the catalyst with, for example, a burner to sufficiently high temperatures such that sulfur oxides are removed from the catalytic material. Temperatures required for regeneration, however, are high, possibly greater than 600° C., and may reduce the catalyst life and fuel efficiency of the internal combustion system.
Nitrogen-based selective catalytic reduction (N-based SCR) is another SCR method that uses a nitrogen-based reductant agent such as ammonia (NH3) or urea. The nitrogen-based reductant agent must be stored in a separate storage tank and must be occasionally replenished. Hence, N-based SCR systems lack the simplicity of HC-SCR systems and require the internal combustion engine operator to monitor and replenish another material consumed in the internal combustion process. Moreover, the nitrogen-based reductant agent may be relatively more expensive and less readily available than the fuel that can be used as a hydrocarbon reductant agent.